Nichols said diet and exercise or genetics may play a role. Or, he added, it may be a matter of timing.

The study provided a snapshot of overweight and obese adults at a point in time. Nichols said if he and his team had followed the study population over an extended period, they may have found that some people develop risk factors very quickly, while others take much longer to do so.

Dr. Carlos Lorenzo, an assistant professor of medicine at the University of Texas Health Science Center in San Antonio, suggested there may be significant variation in this population.

"Obese individuals who are metabolically healthy might represent one end of the spectrum of obesity," said Lorenzo, who was not involved in the study. Identifying people at greater risk for heart disease and diabetes based on their risk factors "may be important for prevention and treatment," he noted.

Endocrinologist Dr. Tracey McLaughlin said there is an "increasing movement" to identify subgroups of overweight and obese people at greater risk of metabolic disease who may benefit from weight loss.

"The jury is still out as to whether healthy overweight individuals benefit from weight loss," added McLaughlin, an associate professor of medicine at Stanford University School of Medicine.

The new study involved 1.3 million overweight and obese adults served by four health systems in 11 states and the District of Columbia. Using each person's weight and height, researchers calculated body mass index (BMI), an estimate of body fat.

The large sample size allowed researchers to categorize obese adults by the severity of their obesity.

The study excluded people who already had diabetes. Nichols said that may explain why black adults, who are known to be at greater risk for diabetes than whites, were 28 percent less likely than whites in the study to have risk factors.

Across all overweight and obese adults in the study, the presence of risk factors varied widely. But with increasing levels of obesity, the likelihood of having at least one risk factor also increased.

Among participants who were overweight, 18.6 percent had no risk factors, but among obese participants, almost 10 percent had no risk factors. Among those considered morbidly obese, less than 6 percent had no risk factors, according to the study.

Nichols said additional research is needed to understand who's most at risk and whether the measures used to assess risk factors are appropriate across different ages, races and ethnicities.

"If you're struggling with obesity and you don't have any of these risk factors, keep doing what you're doing," he said. "But don't assume that your health is really any better. You still need to think about diet and exercise."

The study appears in the March issue of Preventing Chronic Disease, an online journal of the U.S. Centers for Disease Control and Prevention.

News stories are written and provided by HealthDay and do not reflect federal policy, the views of MedlinePlus, the National Library of Medicine, the National Institutes of Health, or the U.S. Department of Health and Human Services.